Crystal oscillators are used as stable frequency sources in a wide range of applications including RF, digital circuits, etc. Crystal oscillators, which are comprised of a crystal generally manufactured from quartz and an active amplifier that provides a negative resistance to perpetuate the oscillation, are widely used because they provide a relatively accurate clock/frequency source. They typically provide a better long-term quality clock than other available clock sources such as ceramic resonators, LC tank circuits, etc. Crystal oscillators, however, are not perfect and often it is necessary to tune their output frequency in order to meet system requirements.
The tuning of the output frequency of an oscillator circuit based on a quartz crystal frequency source is typically achieved by a device called a varactor. Varactors are diodes operated reverse biased so no current flows, but since the width of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. In CMOS processes, varactors are formed by placing a heavily positively-doped region (called a P+ implant) inside a lightly positively-doped region (called a PWELL). The capacitance of these junctions behaves similarly to that of an NMOS transistor, which has an N+ implant inside a lightly negatively-doped region (called an NWELL), which also forms a P-N junction device.
In typical prior art oscillators, a single varactor is used which makes tuning the oscillator a relatively simple procedure. Alternatively, an array of capacitors arranged in a matrix can be used as the tuning element.
A block diagram illustrating a prior art digitally controlled crystal oscillator (DCXO) incorporating a varactor matrix is shown in FIG. 1. The DCXO circuit, generally referenced 10, comprises an external quartz crystal 12, an oscillator circuit 14 and varactor matrix 16. In this example, the quartz crystal 12 is connected to a one transistor oscillator called a Colpitts oscillator. The oscillator is digitally controlled whereby the center frequency of the oscillator output 20 is adjusted by adding parallel capacitance to the circuit such that it would be reflected directly or indirectly to the crystal. This causes slight changes in the output frequency of the oscillator. Since the physical properties of the quartz crystal are fixed, the range of adjustment of the center frequency is relatively small, on the order of 10-20 parts per million (ppm).
In the case of an array of capacitors 16, a digital tuning command 18 is used to determine the total capacitance applied to the oscillator circuit. The digital tuning command is translated to row and column decode signals which control the on/off state of all the capacitors in the matrix.
A problem arises in the need to test all the capacitors in the matrix. In the case of a single varactor, testing is rather trivial. With a large matrix of capacitors, however, testing is more difficult. Each capacitor, having a size on the order of femtofarads, represents a very small share of the total ppm, for example 0.01 ppm. If we assume a center frequency of 26 MHz, for example, this translates to a frequency of less than 1 Hz. Thus, testing each individual capacitor for a frequency of less than 1 Hz will require more than 1 second. Considering an array of tens or hundreds or even thousands of capacitors requires more than an hour of testing. This is an astronomical cost for testing the varactor matrix using modern test equipment.
An alternative approach to testing is to probe each individual capacitor. This, however, is also not practical as each capacitor is measured in femtofarads, applying ‘0’ or ‘1’ digital state to each capacitor changes the state by approximately hundreds of electrons. Typical probes have a minimum capacitance themselves on the order of picofarads, thus making them unsuitable for use in measure such capacitor arrays.
Another problem with the DCXO circuit 10 is that a failed capacitor creates nonlinearity in the output frequency versus the digital tuning command. An example of this problem is shown in FIG. 2. The solid line 30 represents the output frequency trajectory versus input code (i.e. tuning command). Assuming for example, the capacitor in the first row, column seven of the matrix is bad. Thus, a change in the input code from 6 to 7 does not yield an increase in output frequency. The expected trajectory of the output frequency is shown in the dotted line 32. Thus, all codes from 7 and up result in incorrect output frequencies.
There is thus a need for a DCXO self test mechanism that is capable of testing an array of capacitors that does not require lengthy and costly testing time. It is also desirable that the self test mechanism provide a capability to overcome the nonlinearities in output frequency caused by failed capacitor elements.